The present invention is generally directed to polymeric implant products. More particularly, the present invention is directed to a polymeric prosthesis for implantation into the body that has been precoated with a bone cement compatible polymer. The bone cement compatible polymer is added to improve the strength of the interface between the prosthesis and a bone cement once the prosthesis is implanted into the body. In general, the present invention is also directed to a process for coating a polymeric prosthesis with a bone cement compatible polymer.
Prosthetic devices are artificial devices used to replace or strengthen a particular part of the body. Such devices can be used in humans or animals to repair or replace diseased or damaged bone, allied tissue associated with the bone, and/or joints associated with the bone. Primarily, prosthetic devices are used to correct or prevent skeletal deformities or injuries and to alleviate the pain and discomfort associated with the deformities or injuries.
When implanting a prosthesis, typically a receiving site or cavity is first prepared in an adjoining bone. In particular, the bone can be cut and reamed out in order to accommodate the prosthesis. A bone cement is then mixed and placed in the receiving site or cavity. A prosthesis is positioned in the bone cement, and the bone cement is subsequently cured and hardened affixing the prosthesis to the bone.
In most applications, bone cement is made from an acrylic polymeric material. Typically, the bone cement is comprised of two components: a dry power component and a liquid component, which are subsequently mixed together. The dry component generally includes an acrylic polymer, such as polymethyl methacrylate (PMMA). The dry component can also contain a polymerization initiator such as benzoyl peroxide, which initiates the free-radical polymerization process that occurs when the bone cement is formed.
The liquid component, on the other hand, generally contains a liquid monomer such as methyl methacrylate (MMA). The liquid component can also contain an accelerator such as an amine (e.g., N,N-dimethyl-p-toluidine). A stabilizer, such as hydroquinone, can also be added to the liquid component to prevent premature polymerization of the liquid monomer.
When the liquid component is mixed with the dry component, the dry component begins to dissolve or swell in the liquid monomer. The amine accelerator reacts with the initiator to form free radicals which begin to link monomer units to form polymer chains. In the next two to four minutes, the polymerization process proceeds changing the viscosity of the mixture from a syrup-like consistency (low viscosity) into a dough-like consistency (high viscosity). Ultimately, further polymerization and curing occur, causing the cement to harden and affix a prosthesis to a bone.
Once implanted, a prosthetic device ideally closely assimilates the characteristics of the bone and/or the joint that the device is intended to repair or replace. The implanted prosthetic device should be capable of supporting and withstanding stresses and strains normally imparted to the repaired or replaced bone joints.
The above process for implanting a prosthetic device is generally accepted within the art and has proven to be a successful process for repairing or replacing damaged bones, bone joints and the like. Prosthetic devices, however, can be prone to loosen within the bone cavity over time. In particular, the acrylic bone cement, which is neither as strong nor as viable as bone tissue, has been universally considered the weakest link in the implant design. It has been found that the bone cement can break away from the prosthesis, can break away from the bone, or can develop stress or fatigue cracks when repeatedly exposed to the normal stress and strains supported by the bones.
Due to these problems, attempts have been made to improve the mechanical properties of prosthetic devices and of the cement interface that exists between the device and the bone. For instance, U.S. Pat. No. 4,491,987, which was filed by the current inventor and which is incorporated herein in its entirety by reference, discloses an improved prosthesis and process for orthopedic implantation of the prosthesis. The current inventor""s prior patent is generally directed to a prosthesis precoated with a polymeric material that is compatible with bone cement. Once implanted, the precoat provides a stronger interfacial bond between the bone cement and the prosthesis.
The present inventor""s prior work provided great advances in the art with respect to the implantation of orthopedic devices, namely orthopedic devices made from metals such as stainless steel, titanium, and cobalt chrome alloys. However, although metallic devices have achieved relatively high degrees of success in repairing joints, these devices are not always well suited for every application. For instance, in some applications, it is preferred to use a more flexible and less rigid material than metal for opposing joint structures. Specifically, polymeric prosthetic devices are particularly well suited for use in replacing the acetabular cup in a hip replacement and replacing the tibia plateau in knee replacements.
Unfortunately, high strength polymeric materials, such as ultra high molecular weight polyethylene (UHMWPE), do not adhere well to conventional bone cement materials. Thus, in order to attach polymeric prosthetic devices to an adjoining bone using bone cement, deep grooves have been formed into the prosthetic devices for forming a mechanical interlock with the bone cement.
Applicant""s copending application teaches the use of a pretreatment of the prosthesis with a mixture of a solvent and a monomer, the monomer being the same as the polymer component of the bone cement. A precoat of a bone cement compatible polymer is then applied to the prosthesis. The coating of the bone cement compatible polymer polymerizes with the monomer bonded to the prosthesis and ultimately forms a copolymer between the prosthetic polymer and the precoat polymer.
In other prior art constructions, polymeric prosthetic devices include a metal backing and stem for bonding the devices to a bone using a bone cement. Alternatively, the polymeric devices have been installed into a bone without cement using bone screws. Bony tissue ingrowth has also been proposed in the past as a means for joining a prosthesis to bone.
While the above described methods and constructions for polymeric prosthetic devices have met with varying degrees of success, there remains much room for improvement and variation within the art. Thus, a need exists for a process for implanting a polymeric prosthesis into a prepared area of the body. More particularly, a need exists for a process that strengthens the interface between a bone cement and a polymeric prosthesis for decreasing the likelihood that the prosthesis will loosen and break away from the cement over time. Further, a need also exists for a precoated polymeric prosthesis that will readily adhere to a curing bone cement mixture once implanted into an adjoining bone.
The present invention provides further improvements in prior art constructions and methods.
Accordingly, it is an object of the present invention to provide a process for precoating a polymeric prosthesis with a bone cement compatible polymer.
It is another object of this invention to provide a precoating process for a polymeric prosthesis which does not require the use of a polymer solvent.
It is another object of the present invention to provide an implant product including a polymeric prosthesis that has been precoated with a bone cement compatible polymer.
Another object of the present invention is to provide a process for bonding a bone cement compatible polymer coating to a polymeric prosthesis.
Still another object of the present invention is to provide an implant product that buffers the stress transfer from a prosthesis to a bone cement and to an adjoining bone by providing a gradual stiffness gradient from the surface of the prosthesis to the bone cement.
Yet another object of the invention present invention is to provide an implant product that minimizes wear of the acetabular cup or tibia plateau surface opposing the femoral head or condylar metal/ceramic prosthesis. Improved wear is provided by a coating which cross-links the UHMWPE powders prior to molding or sintering the coating layer to the surface of the prosthesis.
These and other objects of the present invention are achieved by providing a process for coating a polymeric prosthesis prior to being implanted into the body. The process includes the steps of providing a prosthesis having a shape configured to be implanted into a prepared area of the body. The prosthesis includes a polymeric portion adapted to be attached to an adjoining bone with a bone cement. A coating of a bone cement compatible polymer is sintered to the polymeric portion of the prosthesis. Specifically, the sintered coating of the bone cement compatible polymer provides a stronger adhesion between the bone cement and the prosthesis.
In one embodiment, the polymeric portion of the prosthesis is made from ultrahigh molecular weight polyethylene UHMWPE. Preferably, the bone cement compatible polymer coating applied to the prosthesis is between about 0.1 to about 2 mm thick, has a substantially pore free outer surface, and is made from a sintered blend of an acrylic polymer and UHMWPE. The acrylic polymer, in one embodiment, is polymethyl methacrylate which is mixed with a UHMWPE powder in the presence of a MMA monomer liquid; boiled; and, vacuum dried. Once dried, the UHMWPE, MMA treated powder is again immersed in a MMA/PMMA stock solution having a dibenzoyl peroxide initiator. The resulting mixture is again dried to a powdered end product. The powdered end product is then used to coat a desired area of the prosthesis followed by sintering under a combination of heat and pressure.
The sintered coating has been found to offer an improved surface for binding with conventional bone cements. Further, the coating material lends itself to use in conjunction with useful additives such as reinforcing fibers or separately applied layers of cross-linked UHMWPE powders which impart useful mechanical properties to the installed prosthesis. For example, blended gradients of materials can be formed which offer improved performance and longevity of an installed implant.
These and other objects of the present invention are also achieved by providing an implant product for implantation into the body. The implant product includes an underlying polymeric member having a shape configured to be implanted into a prepared area of the body. The polymeric member defines a surface adapted to be attached to an adjoining bone with a bone cement composition. A coating covers the surface of the polymeric member and is made from a novel activated mixture of UHMWPE and PMMA powders which are sintered with blended powders and result in a molded prosthesis with a bone cement compatible surface layer.
In one embodiment, the polymeric member is polyethylene and the bone cement compatible sintered material comprises a mixture of UHMWPE powder with PMMA powder. As described above, the normally incompatible powders are mixed together with methyl methacrylate in a process which bonds the mixture to a surface of the implant. The sintered mixture layer of UHMWPE with PMMA provides a surface layer having improved properties for bonding with bone cement.
The sintered layer also provides a useful surface which interacts with other materials such as PE fibers or PMMA. Such materials can be added as separate layers or incorporated into compatible mixtures to provide useful gradients of materials, including a cross-linked inner layer of the prosthesis which makes contact with either the femoral head of a hip prosthesis or the condylar portion of a knee prosthesis.
The implant product may be used to replace various bones and bone joints, and is particularly well suited for use as an acetabular cup or a tibia plateau.
Other objects, features and aspects of the present invention are discussed in greater detail below.